1. Field of the Invention
The present invention relates to hysteretic damping for structures used in light-framed constructions, and in particular to a lateral bracing system constructed to provide a high degree of energy dissipation through hysteretic damping along with high initial stiffness so that energy is dissipated at low force thresholds within a light-framed construction.
2. Description of the Related Art
Shear stresses due to natural phenomena such as seismic activity and high winds can have devastating effects on the structural integrity of light-framed constructions. Lateral forces generated during such natural phenomena may cause the top portion of a wall to move laterally with respect to the bottom portion of the wall, which movement can result in damage or structural failure of the wall and, in some instances, collapse of the building.
In constructions such as residences and small buildings, lateral bracing systems were developed to counteract the potentially devastating effects of shear stress on the structural integrity of light-framed constructions. Although various designs are known, typical lateral bracing systems include vertical studs spaced from each other and affixed to horizontal top and bottom plates. The bottom plate is typically anchored to the floor diaphragm or foundation. The bracing system typically further includes sheathing affixed to the studs, upper plate and/or lower plate to increase structural performance under lateral forces. The sheathing used may be oriented strand board (OSB) or plywood, but fiberboard, particleboard and drywall (gypsum board) are also used.
Alternatively or additionally, light-framed construction wall sections may include lateral bracing systems in the form of prefabricated shearwalls. Shearwalls within wall sections of light-framed constructions provide lateral stability and allow the lateral forces in the wall sections to be transmitted from the upper portions of the wall through the shearwalls to the floor diaphragm or foundation of the building where they are dissipated without structural effect on the wall or building.
Many conventional lateral bracing systems perform well initially under lateral loads, but yield and fail upon the repetitive lateral loads which often occur during significant seismic activity and high winds. Upon appreciable yield or failure of the lateral bracing system, the entire system must be replaced.
It is known to provide conventional high strength walls that are capable of withstanding significant lateral loads that occur during seismic and other events. However, such walls place high demands on foundation anchorage and the foundation itself. Namely, the holdown bolts and foundation must also be made strong enough to withstand the large forces transmitted from the wall as they are dissipated through the holdown bolts and into the foundation. Therefore, while stronger walls conventionally perform better under the seismic activity and other loads, conventional design requirements attendant stronger walls cascade throughout the entire structure, requiring stronger foundation anchorage and stronger foundations.
A further difficulty with conventional lateral bracing walls is that the corners of such walls tend to bind against their support surfaces under lateral loads. FIG. 1 shows a conventional shearwall 20 mounted at its bottom on a support surface 22 and at its top to a pair of top plates 24. A lateral force F as shown will result in a downward force F1 at point A and an upward force F2 at point B. Under high lateral loads, these upward and downward loads can damage the wall 20 and/or the support structures above and below the wall.